<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div >
        <div id="app">
             <p>
                 <input type="text" v-model='firstname' placeholder="姓">
             </p>
             <p>
                <input type="text" v-model='lastname' placeholder="名">
            </p>
            <p>
                <input type="text" v-model='fullname' placeholder="全名">
            </p>
        </div>
 
    </div>
    <!-- 开发环境版本，包含了有帮助的命令行警告 -->
    <script src="https://cdn.jsdelivr.net/npm/vue@2/dist/vue.js"></script>
    <script>
   
        var vm = new Vue({
            // 监管的容器 
            el:'#app',
            // 数据定义
            data:{
                firstname:'',
                lastname:'',
                fullname:''
            },
            watch:{
                firstname:function(val) {  //函数内置
                    this.fullname = val +' ' + this.lastname
                },
                lastname:function(val) {
                    this.fullname = this.firstname + ' ' + val
                }
            }
        })
     
     

      

    </script>
</body>

</html>